Borates have been used as broad-spectrum wood preservatives for over 50 years. Their benefits include efficacy against most wood destroying organisms such as fungi, termites and wood-boring beetles, coupled with a low acute mammalian toxicity and low environmental impact. Soluble borates such as boric acid, borax and disodium octaborate tetrahydrate are well accepted as aqueous-based preservative systems for treating solid wood products for use in protected environments such as interior building applications and painted external joinery. However, because they are readily leached from treated wood when exposed to moisture, such as can occur in exterior or ground contact applications, they are not generally suitable for use in such exposed environments.
Copper chrome arsenate (CCA) is a leach-resistant wood preservative which has been used for may years to treat solid wood for exterior applications. However, such preservatives are facing increasing regulatory pressure as a result of environmental, health and safety problems due to the toxic nature of arsenic and chromium and suitable alternative systems have long been sought. Copper chrome boron (CCB) systems have been used, however the borate in these systems is leachable and over time all the borate will be removed, and the treated timber components will eventually decay, typically due to copper tolerant fungi. If the leaching of the borate components could be reduced in such systems, they would out-perform traditional preservatives such as CCA, as borate is a more effective fungicide than copper or zinc in the absence of leaching.
Solid zinc borate is added to wood composites during manufacture, because its inherent low solubility reduces leaching of the preservative in high moisture environments. However, such low solubility borates are not readily applied to solid lumber. Dev et al. (J. Timb. Dev. Assoc., 1997) described a two-stage process for treating solid wood with zinc borate in which the wood is treated with solutions of borax and zinc chloride in two separate steps. However, the high cost of retreating and rehandling the wood makes the commercial use of such multi-stage processes unattractive.
Ammonia-based solutions have been proposed to solubilize metals such as zinc and copper in an attempt to fix borates in wood. U.S. Pat. No. 2,194,827 discloses an aqueous ammonia solution of copper, zinc and borate salts for the treatment of wood. Similarly U.S. Pat. No. 2,573,253 discloses a cupric ammonium borate solution which may be used for the preservation of wood. However, in order to achieve a high concentration of metals and borate in such solutions, it is necessary to use high concentrations of ammonia, resulting in excessive ammonia volatility and noxious fumes, creating undesirable worker exposure problems in large-scale operations.
JP Patent No. 56025363 discloses wood preservative compositions containing copper and zinc formate or acetate in combination with boric acid, sodium fluoride, starch and triethanolamine. Such amines can present problems as they allow more borate leaching and encourage mold growth on treated timber.
UK Application No. GB 2,187,096A discloses a preservative for wood made up of copper acetate and zinc acetate and optionally also containing a limited amount of boric acid (not more than 10%). However the boron is not effectively fixed and high levels of boric acid are avoided as it may reduce the leaching resistance of the metals.
WO 95/27600 discloses a preservative for treating wood including one or more copper salts and perhaps zinc salts of weak organic acids, an ammonium salt of a weak organic acid possibly mixed with an alkali metal salt of a weak organic acid, nitrite, plus perhaps other chemicals which may include boric acid and quaternary ammonium salts. Increased nitrite concentration improves fixation of the metals, but also tends to promote decomposition of the ammonium salt.